Fuel ethanol is currently produced from feedstocks such as corn starch, sugar cane, and sugar beets. However, the potential for production of ethanol from these sources is limited as most of the farmland which is suitable for the production of these crops is already in use as a food source for humans. Furthermore, the production of ethanol from these feedstocks produces greenhouse gases because fossil fuels are used in the conversion process.
The production of ethanol from cellulose-containing feedstocks, such as agricultural wastes, grasses, and forestry wastes, has received much attention in recent years. The reasons for this are that these feedstocks are widely available and inexpensive and their use for ethanol production provides an alternative to burning or land filling lignocellulosic waste materials. Moreover, a byproduct of cellulose conversion, lignin, can be used as a fuel to power the process instead of fossil fuels. Several studies have concluded that, when the entire production and consumption cycle is taken into account, the use of ethanol produced from cellulose generates close to nil greenhouse gases.
The three primary constituents of lignocellulosic feedstocks are cellulose, which comprises 30% to 50% of most of the key feedstocks; hemicellulose, which comprises 15% to 35% of most feedstocks, and lignin, which comprises 15% to 30% of most feedstocks. Cellulose and hemicellulose are comprised primarily of carbohydrates and are the source of sugars that can potentially be fermented to ethanol. Lignin is a phenylpropane lattice that is not converted to ethanol.
Cellulose is a polymer of glucose with beta-1,4 linkages and this structure is common among the feedstocks of interest. Hemicellulose has a more complex structure that varies among the feedstocks. For the feedstocks which are typically of interest, the hemicellulose typically consists of a backbone polymer of xylose with beta-1,4 linkages, with side chains of 1 to 5 arabinose units with alpha-1,3 linkages, or acetyl moieties, or other organic acid moieties such as glucuronyl groups.
The first process step for converting lignocellulosic feedstock to ethanol involves breaking down the fibrous material. The two primary processes are acid hydrolysis, which involves the hydrolysis of the feedstock using a single step of acid treatment, and enzymatic hydrolysis, which involves an acid pretreatment followed by hydrolysis with cellulase enzymes.
In the acid hydrolysis process, the feedstock is subjected to steam and a mineral acid, such as sulfuric acid, hydrochloric acid, or phosphoric acid. The temperature, acid concentration and duration of the acid hydrolysis are sufficient to hydrolyze the cellulose and hemicellulose to their monomeric constituents, which is glucose from cellulose and xylose, galactose, mannose, arabinose, acetic acid, galacturonic acid, and glucuronic acid from hemicellulose. Sulfuric acid is the most common mineral acid for this process. The sulfuric acid can be concentrated (25-80% w/w) or dilute (3-8% w/w). The resulting aqueous slurry contains unhydrolyzed fiber that is primarily lignin, and an aqueous solution of glucose, xylose, organic acids, including primarily acetic acid, but also glucuronic acid, formic acid, lactic acid and galacturonic acid, and the mineral acid.
In the enzymatic hydrolysis process, the steam temperature, mineral acid (typically sulfuric acid) concentration and treatment time of the acid pretreatment step are chosen to be milder than that in the acid hydrolysis process. Similar to the acid hydrolysis process, the hemicellulose is hydrolyzed to xylose, galactose, mannose, arabinose, acetic acid, glucuronic acid, formic acid and galacturonic acid. However, the milder pretreatment does not hydrolyze a large portion of the cellulose, but rather increases the cellulose surface area as the particle size of the fibrous feedstock is reduced. The pretreated cellulose is then hydrolyzed to glucose in a subsequent step that uses cellulase enzymes. Prior to the addition of enzyme, the pH of the acidic feedstock is adjusted to a value that is suitable for the enzymatic hydrolysis reaction. Typically, this involves the addition of alkali to a pH of between about 4 and about 6, which is the optimal pH range for cellulases, although the pH can be higher if alkalophilic cellulases are used.
In addition to cellulose, hemicellulose, and lignin, lignocellulosic feedstocks contain many other organic and inorganic compounds. Among the most common inorganic compounds are salts of calcium. It is desirable to remove calcium from the process streams, because salts such as calcium sulfate have a low solubility in water and can therefore precipitate on process equipment. Such precipitation can decrease the efficiency of a process and can cause a unit operation or a plant to shut down to remove it.
During the processing of lignocellulosic feedstocks to ethanol, other inorganic salts are produced that can potentially be recovered and sold as commercial products. Recovering these salts is advantageous in that it provides a source of revenue for the plant and offsets the cost of the chemicals used during the chemical processing steps. Of particular value are sulfate salts, including potassium sulfate, sodium sulfate and ammonium sulfate, as they find use as agricultural fertilizers. Alternatively, in regions where fertilizer usage is limited, ammonium sulfate salt recovered from the process may be decomposed to produce sulfuric acid and sulfate salt, which may then be recovered for use in earlier stages of the process or for sale as commercial products as described in co-pending U.S. application No. 60/824,142 (Curren et al.).
Sulfate salts can arise at various stages of processing of the lignocellulosic feedstock. For example, sulfate salts of potassium, calcium, magnesium and sodium are formed during pretreatment by reaction of the sulfuric acid with salts present in the feedstock, while sulfate salts of ammonium, sodium, or potassium are produced at high concentrations upon neutralization of the sulfuric acid present in the pretreated feedstock with ammonium hydroxide, sodium hydroxide, or potassium hydroxide, respectively, prior to cellulase hydrolysis. Sulfate salts may also arise in process streams obtained from strong acid hydrolysis with sulfuric acid.
In order effectively to utilize sulfate salts as a fertilizer, or for other applications, it is first necessary to separate them from other components of the sugar stream. In this connection, it has been proposed to subject sugar streams containing sulfate salts to ion exclusion as disclosed by WO 2005/099854 (Foody et al.). This separation technique uses ion exchange resins with the charge on the resin matching that of the target ions in the solution, thereby excluding them from the resin. The excluded compounds then elute from the column readily, while uncharged compounds absorb into the resin and elute from the column more slowly. The method of Foody et al. (supra) involves the separation of sulfate salts by ion exclusion from an aqueous process stream containing glucose, xylose and arabinose sugars obtained from sulfuric acid pretreatment. In particular, a salt raffinate stream containing sodium sulfate, potassium sulfate, magnesium sulfate, and possibly calcium sulfate and a separate sugar product stream, which contained the vast majority of the organic compounds, were obtained from the process. The Foody et al. process does not separate calcium sulfate from the other sulfate salts. Therefore, further processing of the salt raffinate stream would run the risk of precipitation of calcium sulfate. In addition, the use of ion exclusion by Foody et al. is inefficient, in that it requires large equipment to carry out ion exclusion with several hours of liquid residence time. Ion exclusion also requires the addition of large amounts of water to desorb organic compounds from the resin. This results in a high degree of dilution of the sugar and salt streams.
The isolation of potassium sulfate from process streams by crystallization is known as disclosed by U.S. Pat. No. 5,177,008 (Kampen). In particular, the process involves fermenting the raw material, collecting the ethanol by distillation and then crystallizing the potassium from the remaining still bottoms. However, since Kampen et al. used sugar beets, they were able to crystallize potassium sulfate directly from the still bottoms. By contrast, acid pretreatment of lignocellulosic feedstocks results in mixtures of inorganic salts in the still bottoms that cannot be directly crystallized.
Another method of removing inorganic salts from process streams is ion exchange, which involves the exchange of cations or anions in an aqueous stream with cations or anions on the resins, followed by a subsequent regeneration step to displace the adsorbed species and regenerate the resin. During cation exchange, the resin binds the cations in the feed stream, while neutral compounds, such as sugars and acids, pass through the column in a low-affinity stream. After a certain volume of the process stream has been fed, the resin is saturated and is then regenerated. This is then accomplished using a regenerant solution, which is passed through the resin to convert the cation exchange resin back to its original form. This produces salts from the cations adsorbed to the resin. For example, when hydrochloric acid is used as a regenerant, the resin is converted to the hydrogen form. Soluble chloride salts are formed in the regeneration stream upon reaction of the hydrochloric acid with adsorbed cations.
It is known to demineralize sugar solutions by ion exchange during sugar refining processes to remove ionic impurities (See, for example, U.S. Pat. Nos. 5,443,650, 4,329,183, 6,709,527, 4,165,240, 4,140,541, 5,624,500 and 5,094,694). In particular, these demineralization processes involve passing the sugar solution through a strongly acidic cation exchange resin to remove cationic impurities, followed by passage through a strongly basic anion exchanger to remove anions in a similar manner. The regeneration streams from the ion exchange operations may optionally be utilized in fertilizers as disclosed, for example, in U.S. Pat. Nos. 6,709,527, 4,140,541, 6,709,527 and 5,624,500.
German Patent No. 2418800C2 (Meleja et al.) discloses a process employing ion exchange to purify a hemicellulose hydrolyzate obtained from an acid treatment of beech wood chips. The process involves first hydrolyzing the chips with sulfuric acid, followed by rinsing with water, removal of the pulp from the hydrolyzate and neutralization of the hydrolyzate with sodium hydroxide. The neutralized hydrolyzate was reported to contain Na2SO4, as well as xylose and other sugars resulting from the hydrolysis. The hydrolyzate was then heated and subjected to desalination and ion exchange cleaning steps by passing the solution through successive beds of a strong cation exchanger. The sugar fraction, which contained primarily xylose, and small amounts of Na2SO4, was subsequently subjected to a further cleaning step by running the solution through the successive beds of a decolorizing resin, a strong cation exchanger and a weak anion exchanger. The process disclosed resulted in a xylose solution which was of sufficiently high purity to obtain a high-purity xylitol solution from catalytic hydrogenation of the xylose. However, there is no disclosure of methods for recovering the sulfate salts from the process; rather, the process is directed to producing xylose as the product of the separation.
It is known to demineralize sugar solutions by treating them with cation exchange resins using sulfuric acid as a regenerant. The use of sulfuric acid as a regenerant is particularly advantageous in that it is inexpensive and produces high-value sulfate salts. Such a process is disclosed in a paper by Kearney and Rearick which involves softening sugar beet juice using a weak cation exchange process. (Entitled “Week Cation Exchange Softening: Long Term Experience and Recent Developments” (ASSBT 2003) Published in Proceedings from the 32nd Biennial ASSBT Meeting, Operations, San Antonio, Tex., Feb. 26-Mar. 1, 2003). During regeneration of the cation exchange resin, the sulfuric acid regenerant is converted to calcium sulfate, which is then re-used in an earlier stage in the processing of the sugar beets referred to therein as “pulp pressing”.
Similarly, U.S. Pat. No. 4,046,590 discloses a process for producing a colourless, low-ash, high-purity sugar syrup from cane molasses involving cation exchange with a regenerant solution of sulfuric acid. In particular, the process involved subjecting acidified cane molasses to ion exclusion, de-ashing with cation exchange using sulfuric acid as the regenerant, followed by removal of anions by anion exchange.
However, a disadvantage of processes employing sulfuric acid as a regenerant during cation exchange is that CaSO4 produced during the regeneration has a very low solubility of around 2 g/L, the precise value depending on the temperature and pH. With the use of sulfuric acid regenerant solutions of 20 to 150 g/L, it is likely that CaSO4 forms and precipitates within the resin bed and in the cation exchange equipment. These precipitates interfere with the ion exchange process and the flow of feed onto the column, and are difficult and expensive to remove from the resin bed.
Thus, to date, there has not been an effective method for removing calcium and obtaining sulfate salts from sugar streams resulting from the processing of lignocellulosic feedstocks. The removal of calcium avoids problems with calcium precipitation in downstream processes. The ability to recover the sulfate salts from sugar solutions represents a large opportunity to avoid the cost of their disposal and can lower process costs by providing a product that can be sold as a fertilizer or used for other applications.